1. Field of the Invention
The invention relates to an illumination system with a light source and a field plane, comprising at least one collector with at least one mirror shell which is arranged in a rotationally symmetrical way around an axis for receiving a part of the light of a light source, so that an aperture diaphragm plane arranged in the light path after the collector is illuminated in an annular way.
2. Description of the Related Art
A projection exposure system is known from U.S. Pat. No. 5,737,137 which comprises a laser plasma source as a light source. The illumination system of the projection exposure system according to U.S. Pat. No. 5,737,137 is arranged in such a way that an exit pupil of the illumination system is illuminated in a circular (i.e. annular) fashion. In order to achieve this, the illumination system comprises an arrangement of three mirrors, with two mirrors forming an inverse Cassegrain or Schwarzschild system with a 25% obscuration. The mirrors of the Schwarzschild system are normal-incidence mirrors with a multi-layer system on which the light rays impinge under angles α<30°.
The projective objective of the projection exposure system as known from U.S. Pat. No. 5,737,137 comprises two mirrors with an aperture through which the light passes through.
Moreover, the system according to U.S. Pat. No. 5,737,137 is a system with critical illumination.
The disadvantageous aspect in the system according to U.S. Pat. No. 5,737,137 is that only normal-incidence mirrors are used in the illumination system directly downstream of the source. Although the entry of debris into the projection objective system can be prevented according to U.S. Pat. No. 5,737,137, the normal-incidence mirrors of the illumination system will degrade very rapidly by the debris of the source. As a result of degradation of the multi-layer system, normal-incidence mirrors lose reflectivity to a very high extent over time.
A further disadvantage of the system according to U.S. Pat. No. 5,737,137 is that for a homogeneous illumination of the field plane at critical illumination with a small source it is necessary to have a very large lateral magnification. In the case of very large magnifications however, the dimension especially of the first mirror will become very large and thus difficult to produce with an inverse Cassegrain or Schwarzschild system.